Most people welcome the arrival in April and May of warm weather, the blossom on the trees, the smell of the first cut of lawn and the chance to relax outdoors. But for millions of Britons the sound of the first cuckoo of spring is an air-raid warning signalling the start of the hay fever season.

The sight of grass beginning to grow tall and green again after winter and fields of bright yellow oilseed rape mean runny, blocked noses, disturbed sleep, itchy eyes, mouths and ears, and a fatigue that never leaves.

Jane Bell and daughter Lilly-Ann, 10, are among the 12 million people laid low by hay fever. Six weeks ago she and husband Michael mistook their 11-month-old son Albert's itchy nose and streaming, bloodshot eyes for a cold. But it turned out that he, too, has succumbed to hay fever. 'Lilly's allergic to nuts, dairy, soya and eggs, as well as both tree and grass pollen; she's hypersensitive. She spends part of the year with skin rashes or swollen eyes and ends up with disturbed sleep for six months. We know the hay fever season is starting when she gets bright red eyes and her nose streams,' says Jane, 37.

'The hay fever affects her a lot. In the last three weeks she's been bad, has felt lethargic and also been getting up in the night to ask for medicine. She often says "I hate the summer" because of the hay fever and the medicine she has to get.'

For Lilly-Ann, her condition is doubly frustrating because it is making revision for her Sats exams more difficult. 'She wants to do well but is feeling like rubbish,' says Jane. 'She finds having hay fever incredibly tiring and uncomfortable; everything is hard work. And she wheezes and snores at night, which makes her sister Heidi complain because they share a room.'

Doctors have told Jane that the children's allergies are probably her fault. 'They think the allergy problems are genetic, that the children have inherited this from me.' While scientists and doctors are making strides in their knowledge of allergies, plenty is still not understood. One in three of us now suffers from an allergy, and the number is rising all the time, the House of Lords science and technology committee found last year. The substances responsible range from pets and dust mites to latex and many foodstuffs.

'More people seem to be becoming more allergic to more things. Why? Nobody really knows. We can hypothesise and cite as an example the increased cleanliness of so many homes in the Western world, but we don't know for sure,' says Professor Jean Emberlin, director of the National Pollen and Aerobiology Research Unit at Worcester University. Among the many allergy-related illnesses, hay fever is by far the biggest problem. It is the most common allergy in the West. Prevalence is rising in most age groups in the UK. 'Fifteen to 25 per cent of the population get it at some point, and 38 per cent of teenagers have it,' says Emberlin. While the rate of increase in the number of people afflicted by it has slowed, it is still going up.

About 950,000 people a year go to their family doctor about it, of whom about 535,000 are diagnosed as having it for the first time, according to the Royal College of General Practitioners. Accurate numbers are hard to pin down as many sufferers, possibly most, do not bother their GP, heading to a chemist and stocking up with remedies such as antihistamine tablets and syrups, nasal sprays and eyedrops.

High-street chemist Boots has a large share of this booming market. A spokesman said: 'While core times for demand for hay fever products are April to July, sales were up during our mild February this year and our warm April last year meant sales increased then.' While sceptics dismiss hay fever as little worse than a bad cold, sufferers know it can have a hugely negative effect on their life. 'It can affect their work, their sleeping or their school life because they're itching all the time, become very tired and lose concentration,' said Lindsey McManus of Allergy UK.

Schoolchildren waiting to take their exams are among those worst hit. One study has found that children who have hay fever get better results in their mock exams in the winter than when they take the real things in the summer because their eyes are streaming.

Hay fever is triggered by pollen. Unfortunately one of the many side effects of climate change is that both the grass and tree pollen seasons have been getting longer. 'We're getting more and more calls from people affected, and people call us much earlier than before, and the allergy season goes on longer than before,' says McManus. 'The allergy season used to be late March to late September but over the last five years it's become early March to early October.'

A minority of hay fever sufferers are affected only by the pollen from trees such as birch, alder and willow, which is an issue any time between February and June. But for most it is grass pollen which triggers symptoms. Some are vulnerable to both. 'In the UK there are about 10 main types of allergic tree pollen, but far more pollen types,' says Emberlin. 'With grass pollen, all the species have cross-reactions, so if someone is allergic to one type they will be allergic to the others. There are about 150 species of grass in the UK, but 12 main ones contribute most of the airborne grass pollen.'

The fact that pollen triggers these symptoms gives hay fever both its popular and its medical name: seasonal allergic rhinitis. Rhinitis describes an allergic reaction to a substance which causes inflammation and irritation to the lining of the eyes, nose and throat. In trying to rid the body of the pollen, the immune system overreacts and releases chemicals which cause inflammation.

Further evidence of the growing toll of misery is the apparent increase in the number of people who appear to be getting hay fever for the first time in their thirties, forties or fifties. Nobody collects figures showing when people are diagnosed with it for the first time, but most sufferers are diagnosed when they are young, often between five and 14, says the GPs' college. Traditionally, people who have had it since childhood find that the symptoms ease as they grow older.

'While hay fever is less prevalent in older age groups, I've recently met people aged between 30 and 60 who tell me that they have only had hay fever for four or five years and never had it as children. That's quite likely and quite credible,' says Emberlin. 'It could be because people move to a different place with different allergens, such as from the country to London. Or it could be because of air pollution, or because they've just had a baby. Stress can also bring it on for the first time. The physiological mechanisms aren't well understood. My best guess is [it's because of] increasing allergen loads, because grass pollen counts have gone up by 10 to 20 per cent in places like London over the last 10 to 14 years.'

Pollen also poses problems for many of the 4.1 million adult Britons and 1.1 million children who suffer from asthma. About 80 per cent of asthmatics have rhinitis; 40 per cent of those 5.2 million asthmatics also get hay fever. 'Pollen is a bad thing for asthmatics if they are allergic to it,' says Professor Peter Barnes, head of respiratory medicine at Imperial College London.

Baroness Finlay of Llandaff, who chaired last year's Lords inquiry into allergies, said Britain was 'the laughing stock of Europe' because it ignored treatments routinely available in other countries. Her committee called for the creation of a network of specialist allergy centres across the UK to meet the rising demand for proper diagnosis and effective treatment.

Dr Pamela Ewan, head of the Allergy and Clinical Immunology department at Addenbrooke's hospital in Cambridge, says the growing number of people with hay fever are suffering unnecessarily because there are so few such centres. 'The gap between allergy services and patient need is wide,' she says.

She also highlights a more urgent problem: only about a handful of all those people with severe hay fever - whose symptoms cannot be relieved with the usual medications - get the treatment they need because financial constraints put it off-limits to all but a few. Allergy UK says a third of all hay fever sufferers may experience severe symptoms.

Ewan is talking about immunotherapy, which involves giving tree or grass pollen sufferers an extract of the substances that cause them such severe problems. For years, a few such people have been able to get a course of injections or, more recently, Grazax, hailed by some as a 'hay fever wonder drug' but only designed for those afflicted by grass pollen.

The trouble with both is their limited availability. The reason, Ewan believes, is primarily financial. A three-year course of the jabs costs £1,000 and a year's supply of Grazax tablets costs £800. Only a few primary care trusts, which decide which treatments are worth funding, have opted to fund either, despite requests from doctors. 'I suspect they fear that, if they do, the floodgates may open, given how many people may potentially benefit,' she adds.

In reply to such criticisms, the Department of Health would only say: 'The government recognises the importance of allergy and is committed to playing its part in helping to alleviate the burden of allergic disorders. The provision of allergy services is determined locally.

'However, in 2006 the department completed a review of allergy services in England. The evidence presented in our report should help the local NHS to consider the need for improved services, in the light of local needs.'